1. Field of the Invention
The subject invention pertains to an apparatus and method for initiation of a plasma, in particular a plasma initiator and power distribution system.
2. Description of Background Art
High current power processes are recognized as being relatively dirty due to the lack of precise control over power output. Efforts to improve control over high power processes have for the most part been focused on power delivery in welding processes. Various external mechanisms have been devised in attempts to reduce inconsistencies in output. While some success has been achieved in reducing problems associated with mechanical malfunctions, total dynamic control over power output remains a challenge.
Ionic Plasma Deposition (IPD) is a basic technology employed in coating and other processes where a plasma is deposited on a substrate. The method has been adapted to some extent to modify metal surface coatings by controlling the size and density of plasma particles. A metal plasma produced by this method is generated from a cathode target onto a substrate, which acts as the anode. Several methods have been used to initiate the arc, most using some type of mechanical device, which unfortunately tends to be susceptible to high failure rates. The resulting loss in time and productivity are a particular concern in manufacturing operations.
Nano or macroparticle deposition processes are increasingly of interest because of the need to control surface textures and the importance of surface coatings in medical devices. Cell attachment for example is important in certain types of implants, while in other applications, the control of surface texture may directly affect function. In addition to functionality, some commercial products may depend on a blemish free appearance for success in the marketplace. Highly controlled deposition of nanoparticulate surfaces is believed to have the potential to produce superior surfaces free of blemishes and to offer ways to customize functionality based on surface topography. So far, reproducible nanoparticulate surfaces have been difficult, if not impractical, to achieve with current technology.
Nano plasma deposition (NPD), like ionic plasma deposition, requires plasma initiation, arc control and source utilization. Initiation of the plasma arc as currently practiced, typically utilizes a mechanically driven device. An exemplary type of initiator, also termed a trigger or striker, is a rod made of tungsten or similar material connected to the anode, which is connected to a mechanical device that brings the rod in momentary contact with the target. The process produces varying levels of contamination from the plasma trigger material because the trigger is exposed to high temperatures at the target. The vaporized material contaminates the target, substrate(s) and even the vacuum system itself. Failure of the device is a frequent occurrence, making the system generally unreliable.
One type of initiator or striker is described in U.S. Pat. No. 6,936,145. A ceramic striker pre-plated with a thin metallic film is attached directly to the negatively biased target. An input lead is then attached to one end of the striker so that the lead is in contact with the metallic film. The input lead is grounded through a normally “open” mechanical switch controlled by a relay coil. When a timed control power is applied to the relay coil it is momentarily activated and the switch closes. A limiting current, approximately 10 amps, flows through the plated ceramic striker causing an arc to be struck or initiated. The transient current that initiates the arc burns the thin-coated plating/coating away from the striker. While there is no mechanical movement of the striker in this system, a relay on the outside of the vacuum chamber is required to momentarily short it to ground or to the anode. The grounding puts stress on the mechanical relay, frequently causing the relay contacts to weld together. Large amounts of current are then drawn through the contacts, which may cause damage to the relay. Additionally, due to the nature of this circuit, the ceramic has a tendency to heat, break and in some cases, melt.
A “trigerless” initiator is described in U.S. Pat. No. 6,465,793. The device is described as having no moving parts internal to the plasma chamber although there are many external components. The initiator utilizes a stand alone power supply that is different and separate from the power supply to the target used to generate the plasma. Power speed is controlled within specific parameters (one to 300 cycles per second range), but relies on external switching using a mechanical switch that has proven not to be robust.
Most initiators are designed to vaporize the coating on a non-conductive substrate prior to initiation of the plasma in the belief that it is necessary to break contact between plasma and substrate before deposition occurs. Unfortunately, this has the effect of increasing the time required between initiating the plasma. In particular, it is difficult to perform a short deposition or quick on/off pulses using such prior art initiators.
Current nano plasma deposition methods, such as physical vapor deposition (PVD), also lack the ability to control the arc that creates the metallic plasma. Basic PVD, because it has little or no control, deposits macro particle in a large range of sizes and shapes. These particles can range in size from a few angstroms to tens of microns in diameter. Without power control, the plasma generation source can dwell in one spot for an unspecified amount of time. The longer power stays in one spot, the larger the macroparticles produced. The average size of the macroparticles in an uncontrolled power process then becomes primarily dependant on the melting point of the material. Chrome, for example, with a melting point of just above 1900° C. has an average macro particle size of one micron in an uncontrolled process. Aluminum, on the other hand, with a melting point of 660° C., has an average macro particle size of just over 10 microns under the same process conditions.
In general, plasma deposition processes in current practice rely on a trigger that must be physically moved to come into contact with the target, has a high failure rate because of local overheating, fusing of critical weld points or simply is unable to sustain a controlled deposition rate. Most triggers are highly susceptible to breakdown and failure.
Accordingly, plasma initiators in current use are dependent on some type of mechanical device and require isolation of the positive and negative terminals of a plasma source. The mechanical device is normally placed outside the plasma chamber and consists of a mechanical relay that momentarily shorts the positive and negative terminals. Such an arrangement is generally unreliable because several problems can arise after the short occurs and the relay attempts to separate to break the connection.
For example, the contactor may actually fuse, thereby preventing separation of the connection between the positive and negative terminals. This can cause the relay to overheat when the full current, usually in excess of 100 amps, is passed through the contactor, which typically is rated for only 10 amps.
Burning of the contacts can also occur due to an arc drawn between the positive and negative terminals of the plasma source. The arc can foul the contactor and thus the momentary connection needed to initiate the plasma cannot be made without maintenance to the relay.
If the plasma is extinguished or interrupted, it can be re-initiated only if the metal plasma has been depositing for longer than about 30 seconds. The initiator inside the chamber will need to be re-coated by metal because the initiation is dependent on the thin metallic film being vaporized upon initiation.
A lack of effective controls for currently available power control systems is another problem encountered with nanoparticle deposition. While some power controls seem to address to at least some of the deficiencies, the application may be limited. For example, U.S. Pat. No. 5,037,522 describes a method of power control using sensors to detect the power at the end of a cylindrical target. Unfortunately, this technology is viable only using a cylindrical target setup as illustrated in the '522 patent. This limits the use of the power supply since the actual source configuration of the metallic vapors is a factor in controlling nano particle size.
Presently available control systems only address the ability to prevent the power from dwelling in one spot too long. When a reduced macro particle coating is desired, these methods, though not perfect, while adequate for some applications, are unsatisfactory where controlled surface quality is needed. Medical device cell attachment coatings, for example, require not only an actual increase in the number of macroparticles, but also control of the size of deposited macroparticles. The external electronic controls that are currently available do not address this problem.
In general, therefore, the lack of precise control over output power in high current processes has limited development of methods for producing specialized surfaces that require defined surface structuring. While some control over power delivery has been achieved, mostly through the use of external control devices, little attention has been given to total dynamic control over power output. Plasma arc control methods are limited and require complex sensors and other electronics that generally are not sufficiently robust to withstand commercial use. Control of power speed has not been adequately addressed and little attention has been paid to eliminating power hanging in one spot on the target or to preventing the power from jumping around on the target surface.